Along with the recent years' requirement for better fuel-efficiency of automobiles, a conjugated diene-based polymer is demanded which can provide a vulcanized rubber, as a rubber material for tires, having a low rolling resistance, excelling in abrasion resistance and fracture characteristics, excelling in wet skid resistance, and having an excellent rigidity.
In order to reduce the rolling resistance of tires, the hysteresis loss of vulcanized rubber need to be made low. As an evaluation index of the hysteresis loss, rubber materials are preferable which have a high impact resilience at 50 to 80° C., or a low tan δ or Goodrich heat generation at 50 to 80° C. As rubber materials having a low hysteresis loss, natural rubbers, polyisoprene rubbers, polybutadiene rubbers and the like are known, but these have a problem of having a low wet skid resistance.
As methods for reducing the hysteresis loss without impairing the wet skid resistance, there are proposed a method of increasing the 1,2-vinyl structure being a microstructure of butadiene in styrene-butadiene (co)polymers having various structures polymerized by an organolithium initiator in a hydrocarbon solvent, a method of introducing a modifying group to polymer terminals, and other methods.
On the other hand, in order to improve the steering stability of high-performance tires, vulcanized rubbers are desired to have a high rigidity, and as evaluation indexes, they preferably exhibit a small change in storage elastic modulus against strain at 50° C., and a high storage elastic modulus at a high strain. As methods of enhancing the rigidity of vulcanized rubbers, there are no effective methods other than the improvement of filler compositions, and making the molecular weight high as an improvement of raw material rubbers.
Under such a situation, for example, Patent Documents 1 to 4 disclose modified polymers obtained by reacting active terminals of a rubbery polymer with a polyfunctional low molecular weight compound having an epoxy group and a tertiary amino group in its molecule.    Patent Document 1: Japanese Patent No. 2538629    Patent Document 2: International Publication Pamphlet 01/23467    Patent Document 3: Japanese Patent Laid-Open No. 2001-131227    Patent Document 4: Japanese Patent Laid-Open No. 2003-119223